Standards determine whether a given modem can successfully connect with any
other modem in the world.
Bell 103 Uses a kind of modulation called Frequency Shift Keying, or KSK, in
which a specific tone frequency signifies a digital one and another a
digital zero. Each change in the modem's signal thus carries one bit
of digital information. Consequently, the Bell 103 standard is the
only one in which the baud rate (the rate at which signals change) is
equal to the data rate of 300 bps (Bits Per Second).
Bell 212A is the next logical step and the second modem standard to find wide
application in the United States. It achieves a data-transfer rate of
1,200 bps by using phase modulation of a carrier signal. The phase of
a fixed tone, called a carrier wave, is shifted by any of four phase
angles. Under Bell 212A, the carrier wave can change phase up to 600
times per second, or 600 baud. The four phase states are sufficient
to represnet any of four two-bit patterns. Thus each baud can carry
two bits of information, raising the actual throughput to twice the
baud rate of 1,200 bps.
While widely used in America, many foreign countries prohibit the
use of Bell 212A, prefering instead the similar international
standard, V.22.
NOTE: Why do rockwell controllers, in Bell 212A mode require a CCITT V.22 BIS
answer tone?
Cause their controller code is buggy! However, they've shipped a few million of
them, so it's a "standard" now.
LAP-B stands for Link Access Procedure, Balanced, an error-correction
protocol designed for X.25 packet-switched services like Telebit and
Tumnet. Some modem makers adapted it to their dial-up products before
the V.42 standard was agreed upon. For example, the Hayes Smartmodem
9,600, form Hayes Mirocomputer Products, includes LAP-B error-control
capabilities.
LAP-M is an acronym for Link Access Procedure for Modems and is the
error-correction protocol used by the CCITT V.42 standard.
MNP Microcom Networking Protocol is an entire hierarchy of standards,
starting with MNP Class 1, a no longer used error correction
protocol, and running to MNP Class 10, designed to induce the highest
data transfer performance from poor connections, especially those
found in cellular phone systems.
MNP classes 2 though 4 deal with error control and are in the public
domain; classes 5 though 10 are licensed by Microcomm
MNP-2 is designed to work with any modem capable of full-duplex
communications. It works by confirming each byte as it is sent - by
having the receiving modem echo back each character.
MNP-3 improves on MNP-2 by wirking synchronously instead of asyncronously.
As a result, no start and stop bits are required for each byte,
trimming the data-transfer overhead by 25 percent or more.
MNP-4 is an error correcting protocal that also yields some data
compression. It incorporates two innovations. The first, Adaptive
Packet Assembly, allows the modem to package data in blocks or
packets sent and error-checked as a unit, The second, Data Phase
Optimization, eliminates repetitive control bits from the data
traveling across the connection to streamline trasnmissions. Together
these techniques can increase the throughput of a modem by 120
percent at a given bit rate.
MNP-5 is purely a data compression protocol that squeezes some types of
data into a form that transmits faster. MNP-5 can compress up to a
factor of two, effectively doubling the speed of data transmissions.
On files that have been already compressed, however, MNP-5 may
actually increase the transmission time.
MNP-6 is designed to help modems get the most out of telephone
connections, independent of data compression. Using a technique
called Universal Link Negotiation, modems can start communicating at
a low speed, then, after evaluating the capabilities of the teliphone
line and each modem, switch to a higher speed.
MNP-7 is a more efficient data compression algorithm (Huffman encodeing)
than MNP-5, premitting increases in data throughput as much as
threefold on some data.
MNP-8 was never released.
MNP-9 is desinged to reduce the transmission overhead required by some
common modem operations. The acknowledgments of data packets is
streamlined by combining each acknowledgement with the next data
packet instead of sending a separate confirmation byte. While some
error-correction schemes require all information transmitted after an
error to be resent, an MNP-9 modem only requires the incorrect data
to be sent again.
MNP-10 is a set of Adverse Channel Enhancements that help medems work
better with poor connections, compensating for line noise, echo
problems, and limited bandwidth. Modems with MNP-10 will make
multiple attempts to set up a transmission link, optimize the size of
data packets for a connections, and adjust to the highest rate
possible.
V.22 is the CCITT equivalent of the Bell 212A standard, a transfer rate
of 1,200 bps at 600 baud. It uses the same form of modulation as Bell
212A but is not compatible with the bell standard, because it uses a
different protocol to set up the connection. Some modems support both
standards and allow you to switch between them.
V.22bis was the first true world standard, adopted in both the United States
and Europe. It allows a transfer rate of 2,400 bps at 600 baud by
using a technique called trellis modulation that mixes two simple
kinds of modulation; quadrature and amplitude. Each baud has 16
states, enough to code any pattern of four bits. Each state is
distinguished both ny its phase relationship to the unaltered carrier
and its amplitude (or strength) in relation to the carrier. There are
four distinct phases and four distinct amplitudes under V.22bis
V.32 is an international high speed standard that permits data-transfer
rates of 4,800 and 9,600 bps. At 4,800 bps, it uses quadrature
amplitude modulation simiar to Bell 212A, but at 2,400 baud rather
than 212A's 600 baud. At 9,600 bps, it uses trellis modulation
similar to V.22 bis's 600 baud) and with a greater range of phases
and amplitudes.
Note that while most Group III FAX machines and modems operate at
9,600 bps, a FAX modem with 9,600 bps capability isn't necessarily
compatible with the V.32 standard.
V.32bis extends the V.32 standard to 14,400 bps, while allowing intermediary
speeds of 7,200 and 12,000 bps in addition to the 4,800 and 9,600 bps
speeds of V.32.
Note that all of these speeds are multiples of a basic 2,400 baud
rate. The additional operating speeds available to V.32bis are
generated using different ranges of phases and amplitudes in the
modulation. At 14,400 bps, there are 128 potentially different
phase/amplitude states for each baud under V.32bis, enough to encode
seven data bits in each baud. Because there are so many phases and
amplitude differences squeezed together, a small change in the
characteristics of a telephone line might mimic such a change and
cause transmission errors. Consequently, error detection and
correction become increasingly important as transmission speed goes
up.
V.42 is a world wide error correction standard designed to help make
V.32, V.32bis and other modem communications more reliable. V.42
incorporates MNP-4 as an "alternative" protocol. That is, V.42 modems
can communicate with MNP-4 modems, though a connection between the
two won't use the more sophisicated V.42 error correction protocol.
At the beginning of each call, as the connection is being negotiated
between modems, a V.42 modem will determine whether MNBP-4 or full
V.42 error correction can be used by the other modem, MNP-4 being the
second choice.
V.42bis is a data compression protocol endorsed by the CCITT. Different from
and incompatible with MNP-5 and MNP-7, V.42bis is more efficient than
either. On some forms of data, it can yeild compression factors up to
four, potentially quadrupling the speed of modem transmissions. (With
PCs, the effective saximum communication rate is limited by the
serial port itself to no more that 38,400 bps.) Unlike MNP-5, V.42
never slows transmission of "incompressible" data. Worst case
operation is the same speed as would be achieved without compression.

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